Optical head and optical disk device using the same

ABSTRACT

In optical heads with multiple semiconductor laser chips with different wavelengths, coma aberrations generated by laser beams projected at an angle relative to the entry axis of a focus lens is reduced.  
     To achieve this, a beam-shaping upward prism having dispersion characteristics is disposed on the entry side of the focus lens. The semiconductor laser chip with the longer wavelength is positioned closer to a line extending a refracted beam resulting from refraction.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to an optical head and an opticaldisk using the same for recording or playing back information to andfrom an optical information medium such as an optical disk. Morespecifically, the present invention relates to an optical head and anoptical disk using the same that can record information using a lasermodule in which multiple semiconductor laser chips having differentwavelengths are mounted.

[0002] In optical information recording/playback devices such as opticaldisk devices, various features are desired in addition to a compact andthin design.

[0003] For example, there is a significant demand for using a singlecompact optical head that can record and playback both CD-R (CompactDisk-Recordable), which has seen widespread use as a writable opticaldisk medium, and DVD-RAM (Digital Versatile Disc/Digital Video Disc),which was developed recently as an optical disk medium allowinghigh-density recording. The wavelength of lasers used in recording andplayback of CD-Rs is approximately 780 nm, while the wavelength oflasers used in recording and playback of DVDs is approximately 660 nm.Thus, there is a need to mount both a laser light source with a 780 nmwavelength and a laser light source with a 660 nm wavelength on a singleoptical head.

[0004] For example, Japanese laid-open patent publication number Hei10-261240 and Japanese laid-open patent publication number Hei 10-289468propose a compact optical head which integrates into a single unit asemiconductor laser chip with a wavelength of approximately 780 nm forCDs, a semiconductor laser chip with a wavelength of approximately 660nm-for DVDs, and an optical detector element.

[0005] Laser beams emitted from light-emitting points at differentpositions generally pass through different positions of a lens system atdifferent angles. In these optical heads, the laser beams emitted fromthe two semiconductor laser chips enter a focus lens at differentpositions and different angles. In the embodiments described in Japaneselaid-open patent publication number Hei 10-261240 and Japanese laid-openpatent publication number Hei 10-289468, a semiconductor laser chip witha 660 nm wavelength for DVDs is disposed on the optical axis of a lenssystem formed by a focus lens and a collimating lens. A semiconductorlaser chip with a 780 nm wavelength for CDs is disposed away from theoptical axis of the lens system. Since the laser beam for DVDs entersthe focus lens directly from above, the DVD laser spotlight does nottend to generate aberration. On the other hand, the laser beam for CDsenters the focus lens at an angle, and therefore tends to generateaberration (especially coma aberration) in the laser spotlight for CDs.

[0006] In Japanese laid-open patent publication number Hei 10-261240, aholographic optical element is used. In Japanese laid-open patentpublication number Hei 10-289468, an optical means using polarizingprism (a birefringent plate) or holograms allows just the optical pathof the laser beam for CDs to be bent so that it enters straight into thefocus lens.

[0007] To record information, there is also the need for beam-shapingmeans to take a laser beam with anisotropic optical intensitydistribution emitted by a semiconductor laser and efficiently focus itto an optical spot that has an isotropic optical intensity distribution.

[0008] Furthermore, there is a great demand for compact design inoptical heads. Although not described in the embodiments in Japaneselaid-open patent publication number Hei 10-261240 and Japanese laid-openpatent publication number Hei 10-289468, this generally requires opticalcomponents other than the focus lens to be arranged on a plane parallelto the disk surface and an upward projecting mirror to guide the beam tothe focus lens.

SUMMARY OF THE INVENTION

[0009] However, in the conventional technologies described above, it isnecessary to provide special holographic optical elements, polarizingprisms (birefringent plate), and the like that can bend the optical pathof the laser beam with a wavelength of 780 nm for CDs only while notaffecting the laser beam with a wavelength of 660 nm for DVDs. Thisincreases optical component costs in the optical head.

[0010] The object of the present invention is to provide an optical headand optical disk device using the same for recording information orplaying back information to or from an optical information medium usingmultiple laser light sources wherein: aberration of the laser beam fromsemiconductor lasers positioned outside the optical axis are preventedwithout using new, expensive optical components; information can berecorded; and a thin design can be provided.

[0011] In order to achieve this object, a first invention provides anoptical head including: laser light sources emitting a plurality oflaser beams with different wavelengths; means for converting beam widthhaving dispersion characteristics so that the plurality of laser beamsemitted from the laser light sources exits at different angles when theplurality of laser beams enters at identical angles, and converting beamwidths of the plurality of laser beams; and means for optically focusingfocusing the plurality of laser beams exiting from beam width convertingmeans to an optical spot on an optical information medium. The laserlight sources corresponding to the laser beams are positioned in thevicinity of a path of a laser beam projected from an entrance side ofthe beam width converting means when the plurality of laser beams isentered into an exit side of the beam width converting means.

[0012] In the first invention, the laser light sources can be positionedso that the plurality of laser beams emitted from the plurality of laserlight sources enter optical focussing means within an entry angletolerance range. Beam width converting means can be a refraction-typebeam width converting means converting beam widths through refraction.

[0013] A second invention provides an optical head including: laserlight sources emitting a plurality of laser beams with differentwavelengths; means for converting beam width converting beam widths ofthe plurality of laser beams; and means for optically focusing focusingthe plurality of laser beams exiting from beam width converting means toan optical spot on an optical information medium. Beam width convertingmeans has dispersion characteristics so that the plurality of laserbeams emitted from the laser light sources exits at different angleswhen the plurality of laser beams enters at identical angles. The laserlight sources are arranged in a sequence determined by wavelength inorder to reduce shifting in exit angles caused by the dispersioncharacteristics when the laser beams emitted from the plurality of laserlight sources exit from beam width converting means.

[0014] In the second invention, beam width converting means can be arefraction-type beam width converting means converting beam widthsthrough refraction. The plurality of laser light sources can be arrangedso that the laser light sources with longer wavelengths are positionedcloser to an extension line of a refracted beam created by therefraction of beam width converting means. The refraction-typebeam-width converting means can be a prism.

[0015] In a third invention, an optical head includes: a plurality ofsemiconductor laser chips having different wavelengths; a collimatinglens forming parallel beams from a plurality of laser beams emitted fromthe semiconductor laser chips; means for optically focusing focusing theplurality of laser beams on the optical information medium as an opticalspot; and a beam-shaping prism expanding a width of the laser beams in adirection in which the semiconductor laser chips are arranged. Thesemiconductor laser chips with longer wavelengths are positioned closerto an extension line of a beam exiting from the beam-shaping prism.

[0016] In the third invention, the beam-shaping prism can include areflective surface, and semiconductor laser chips with longerwavelengths can be positioned toward a reflective side of saidbeam-shaping prism. Also, the beam-shaping prism can be positioned belowoptical focusing means.

[0017] A fourth invention provide an optical disk device in which alaser beam from an optical head is projected on an optical informationmedium. A laser beam reflected from the optical information medium isprojected onto a plurality of optical detector elements. A signalelectronically converted by the plurality of optical detector elementsis used to provide a control signal and an information playback signal.The optical disk device includes an optical head as described in any oneof claim 1 through claim 9.

BRIEF DESCRIPTION OF DRAWINGS

[0018]FIG. 1 is a top-view drawing of an embodiment of an optical diskdevice according to the present invention, a side-view drawing as seenfrom arrow B, and a side-view drawing as seen from arrow C.

[0019]FIG. 2 is a front-view drawing of an embodiment of a laser moduleaccording to the present invention and a cross-section drawing along theD1-D2 line.

[0020]FIG. 3 is a top-view drawing showing an example of a lens actuatorused in this embodiment and a partial cross-section drawing along theE1-E2 line.

[0021]FIG. 4 is a plan drawing showing an example of a diffractiongrating pattern of a four-part diffraction grating of a compoundelement.

[0022]FIG. 5 is a front-view drawing of an embodiment of a semiconductorsubstrate in the laser module from FIG. 2.

[0023]FIG. 6 is a block diagram showing an embodiment of a signalarithmetic circuit for obtaining a focus offset detection signal, atrack offset detection signal, and an information playback signal.

[0024]FIG. 7 is a perspective drawing of an optical system for thepurpose of describing principles behind an optical head according to thepresent invention.

[0025]FIG. 8 is a plan drawing showing the structure of a beam-shapingupward prism of an optical head according to the present invention.

[0026]FIG. 9 is a perspective drawing showing another embodiment of asemiconductor laser chip.

[0027]FIG. 10 is a side-view drawing showing another embodiment of anoptical disk device according to the present invention.

[0028]FIG. 11 is a plan drawing showing another beam-shaping upwardprism in an optical head according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] An optical head and an optical disk device using the same will bedescribed, with references to the drawings.

[0030]FIG. 1 shows an architecture of an embodiment of an optical diskdevice according to the present invention. FIG. 1(a) is a top-viewdrawing. FIG. 1(b) is a side-view drawing of FIG. 1(a) as seen from thedirection indicated by the arrow B. FIG. 1(c) is a side-view drawing ofFIG. 1(a) as seen from the direction indicated by the arrow C. In FIG.1(a)-FIG. 1(c), elements assigned the same numbers represent identicalelements. The figures show an optical disk 2, representing a CD-ROM diskor CD-R disk having a substrate thickness of 1.2 mm and using a laserwavelength of 780 nm for recording and playback. Alternatively, theoptical disk 2 can be a DVD disk having a substrate thickness of 0.6 mmand using a laser wavelength of 660 nm for recording and playback. Amotor 3 is secured to an optical disk device 1 and rotates the opticaldisk 2 using a rotation shaft 4. An optical head 5 can be moved alongthe radial direction of the optical disk 2 over a rail 7 by an accessmechanism 6, formed from a voice coil motor, pulley, and the like. Theoptical head 5 is equipped internally with a two-laser module 8, acollimating lens 9, a beam-shaping upward prism 10, and a lens actuator11. The two-laser module 8 is equipped with a semiconductor laser chip13 a projecting a 660 nm laser beam 12 a and a semiconductor laser chip13 b projecting a 780 nm wavelength laser beam 12 b. A focus lens 15 anda compound element 14 formed from a quarter-wave plate and a polarizeddiffraction grating are attached to the lens actuator 11.

[0031] Next, the structure of a laser module according to the presentinvention will be described using FIG. 2.

[0032]FIG. 2 shows an architecture of an embodiment of a laser moduleaccording to the present invention. FIG. 2(a) is a front-view drawing.FIG. 2(b) is a cross-section along the D1-D2 line in FIG. 2(a). In thefigures, a package 21 is molded from a material having good thermalconduction such as aluminum nitride. Multiple lead wires 22 are passedthrough the package 21 to transfer electronic signals. A semiconductorsubstrate 24 formed from silicon or the like is disposed inside thepackage 21 and is sealed by the package 21 and a light-transmissiveglass plate 23. An indentation 25 is formed on the semiconductorsubstrate 24 through etching or the like, and a sloped surface of theindentation 25 forms a mirror surface 26 at a 45-degree angle. Thesemiconductor laser chip 13 a and the semiconductor laser chip 13 b aremounted in the indentation 25 and the laser beams 12 a, 12 b are emittedto the right in FIG. 2(b), i.e., in the direction of the mirroredsurface 26. The laser beams 12 a, 12 b are reflected by the mirroredsurface 26 and pass through the glass plate 23 and are projected outfrom the two-laser module 8. The active layers of the semiconductorlaser chip 13 a and the semiconductor laser chip 13 b, i.e., the layersemitting the laser beams, are oriented roughly parallel to the flatsurface of the indentation 25. Thus, when viewed from a position facingFIG. 2(a), i.e., from the direction opposite to the direction in whichthe laser beams 12 a, 12 b are emitted in FIG. 2(b), the opticalintensity distribution of the laser beams 12 a, 12 b forms a roughlyelliptical shape narrow along the vertical axis and wide along thehorizontal axis of FIG. 2(a). The laser beams 12 a, 12 b shown in FIG.2(b) represent the beams before they enter the collimating lens 9.

[0033] In FIG. 1(c), the laser beams 12 a, 12 b exiting from thetwo-laser module 8 are formed into parallel rays by the collimating lens9 and are sent into the beam shaping upward prism 10. The opticalintensity distribution of the laser beams 12 a, 12 b before they enterthe beam shaping upward prism 10 is narrow along the vertical axis ofthe plane of the page of FIG. 1(c) and wide along the axis perpendicularto the plane of the page of FIG. 1(c). The beam shaping upward prism 10is used to make the beam width of the laser beams 12 a, 12 b wider alongthe vertical axis of the plane of the page, providing a more uniformoptical intensity distribution. In other words, the laser beams 12 a, 12b, which are shaped narrow along the vertical axis of the plane of thepage of FIG. 1(c) and wide along the axis perpendicular to the plane ofthe page of FIG. 1(c) before they enter the beam shaping upward prism10, pass through the beam shaping upward prism 10. The vertical lengthof the laser beams 12 a, 12 b varies according to the angle of the entrysurface of the beam shaping upward prism 10 relative to the laser beams12 a, 12 b. Thus, by setting up the angle of the entry surface of thebeam shaping upward prism 10, the laser beams 12 a, 12 b can be providedwith isotropic intensity distribution. The laser beams 12 a, 12 b, whichnow have isotropic intensity distribution, are reflected by the beamshaping upward prism 10 and enter the compound element 14 and the focuslens 15 of the lens actuator 11.

[0034] Next, the lens actuator will be described using FIG. 3.

[0035]FIG. 3 is a drawing showing the architecture of a sample lensactuator used in this embodiment. FIG. 3(a) is a top-view drawing of thelens actuator as seen from the direction of the optical disk. FIG. 3(b)is a partial cross-section drawing along the E1-E2 line from FIG. 3(a).In FIG. 3(b), the optical disk 2 is drawn in for reference. The figuresshow a coil 34, the focus lens 15, and the compound element 14 below it.These are attached to a lens holder 31, which is supported by a supportbase 33 using a spring 32. The solid line 36 in FIG. 3(b) shows thesurface of a case for the optical head 5, to which a magnet 35, thesupport base 33, and the like are secured. The lens actuator 11 providesfocus control by driving the compound element 14 and the focus lens 15vertically along the plane of the page in FIG. 3(b) and also providestracking control by driving the compound element 14 and the focus lens15 vertically along the plane of the page in FIG. 3(a) (along the radiusof the optical disk 2).

[0036] In this embodiment, when the laser beams 12 a, 12 b from thesemiconductor laser chips 13 a, 13 b enter the compound element 14,formed from a polarizing four-part diffraction grating and quarter-waveplate, the beams enter as ordinary rays. In this case, the laser beams12 a, 12 b are passed through the polarizing diffraction grating withoutbeing diffracted and are formed into circular light by the quarter-waveplate in the compound element 14. The laser beams 12 a, 12 b reflectedby the optical disk 2 pass through the quarter-wave plate of thecompound element 17 again to form extraordinary rays, which are thendiffracted by the polarizing four-part diffraction grating.

[0037] The following is a description of the four-part diffractiongrating.

[0038]FIG. 4 shows a plan drawing of a sample diffraction gratingpattern of the four-part diffraction grating in the compound element. Asthe figure shows, a four-part diffraction grating 40 is divided intofour regions by boundary lines 41, 42. A circle 43 indicates the laserbeam 12 a or the laser beam 12 b. The beam is separated by the four-partdiffraction grating 40 into four +1 spectral order beams and four −1spectral order beams. The four regions in the diffraction grating havegrating grooves formed in different directions, but the grooves areequally spaced. Thus, the eight +/−1 spectral order beams have differentdiffraction orientations but the absolute values of the diffractionangles are identical. These eight diffraction beams are focused by thecollimating lens 9 into eight spotlights on the surface of thesemiconductor substrate 24 in the laser module 8 containing thesemiconductor laser chips 13 a, 13 b.

[0039] The following is a detailed description of an embodiment of thesemiconductor substrate 24 in the laser module, using FIG. 5.

[0040]FIG. 5 is a front-view drawing of an embodiment of thesemiconductor substrate in the laser module shown in FIG. 2. Thesemiconductor laser chip 13 a and the semiconductor laser chip 13 b aremounted in the indentation 25 formed on the semiconductor substrate 24.The semiconductor laser chip 13 a beams the laser beam 12 a to the rightin the figure. The laser beam 12 a is reflected at a position 51 a ofthe mirrored surface 26 and exits the surface of the pageperpendicularly. Similarly, the semiconductor laser chip 13 b beams thelaser beam 12 b to the right in the figure. The laser beam 12 b isreflected at a position 51 b of the mirrored surface 26 and exits thesurface of the page perpendicularly.

[0041] In the figure, the eight shaded quarter-circles indicate thespotlights 52 a of the laser beam 12 a reflected by the optical disk 2and separated by the four-part diffraction grating 40. The spotlights 52a lie on the perimeter of a circle having its center at the position 51a. The eight white (unshaded) quarter-circles indicate spotlights 52 bof the laser beam 12 b reflected by the optical disk 2 and separated bythe four-part diffraction grating 40. The spotlights 52 b lie on theperimeter of a circle having its center at the position 51 b.

[0042] Optical detection elements 53-1 a, 53-1 b, 53-2 a, 53-2 b, 53-3a, 53-3 b, 53-4 a, 53-4 b are long, thin optical detection elementsarranged in pairs of facing elements that provide focus offset detectionsignals. The optical detection elements 53-1 a, 53-1 b, the opticaldetection elements 53-2 a, 53-2 b, the optical detection elements 53-3a, 53-3 b, and the optical detection elements 53-4 a, 53-4 b form pairs.These four pairs receive the light from the four spotlights 52 a or thefour spotlights 52 b. Focus offset detection is performed with aknife-edge method (Foucault method) using the four-region beam. A focusdetection signal could be provided by taking the differences of theoutput signals from the pairs of optical detection elements 53-1 a,53-lb, 53-2 a, 53-2 b, 53-3 a, 53-3 b, 53-4 a, 53-4 b to provide a focusoffset detection signal. However, in this embodiment, thelight-receiving elements are connected as shown in the figure byconductive films 54 a, 54 b formed from aluminum or the like. Thedifference between the output signals from an A terminal and a Bterminal of a wire-bonding pad 55 is calculated to obtain a focus offsetdetection signal. Optical detection elements 56 a, 56 b, 56 c, 56 d,which are used to provide a track offset detection signal and aninformation playback signal, are connected to a C terminal, a Dterminal, an E terminal, and an F terminal of the pad 55.

[0043] The signals output from the terminals A-F of the pad 55 are sentto the block shown in FIG. 6 to provide the necessary signals.

[0044]FIG. 6 is a block diagram of an embodiment of a signal arithmeticcircuit providing a focus offset detection signal, a track offsetdetection signal, and an information playback signal. In the figure, adifferential circuit 61 calculates the difference between the outputsignals from the A terminal and the B terminal of the wire-bonding pad55 shown in FIG. 5. The differential circuit 61 outputs a focus offsetdetection signal 62. An adder 63-1 adds the output signals from the Cterminal and the D terminal, and an adder 63-2 adds the output signalsfrom the E terminal and the F terminal. A differential circuit 63-3takes the difference between the output signal from the adder 63-1 andthe output signal from the adder circuit 63-2 and outputs a push-pulltrack offset detection signal 64 for cases when an optical disk havingguide grooves or the like is used. An adder 63-4 adds the output signalfrom the adder 63-1 and the output signal from the adder 63-2 andoutputs an information playback signal 65. An adder 66-1 adds the outputsignals from the C terminal and the E terminal. An adder 66-2 adds theoutput signals from the D terminal and the F terminal. A differentialcircuit 66-3 takes the difference between the output signal from theadder 66-1 and the output signal from the adder 66-2. An output signal67 thereof is used to provide a phase-difference track offset detectionsignal for optical disks that use guide pits or the like. The focusoffset detection signal and the track offset detection signal are sentto a coil 34 of a lens actuator 11 shown in FIG. 3 to drive the focuslens 15 attached to the lens actuator in the direction of the opticalaxis as well as along the disk radius, thus providing automatic focuscontrol and tracking. As a result, the optical intensity of the laserbeam 12 a or the laser beam 12 b from the semiconductor laser chip 13 aor the semiconductor laser chip 13 b can be modulated by an informationrecording signal to allow information to be recorded to the optical disk2. Also, by keeping a constant optical intensity for the laser beam 12 aor the laser beam 12 b from the semiconductor laser chip 13 a or thesemiconductor laser chip 13 b, information recorded on the optical disk2 can be played back using the information playback signal 65.

[0045] The following is a description of the principles behind theoptical head of the present invention, with references to FIG. 7.

[0046]FIG. 7 shows perspective drawings of optical systems for thepurpose of describing examples of optical head principles in the presentinvention. FIG. 7(a) shows an optical system of an optical head having abeam-shaping prism. A semiconductor laser chip 71 a beams a laser beam72 a with a wavelength of, for example, approximately 660 nm. The laserbeam 72 a is made to form parallel rays by a collimating lens 73 and isrefracted by the beam-shaping prism 74, causing the beam width to bewider along the axis going into the plane of the page. This beam thenenters a focus lens 75. If the semiconductor laser chip 71 a is replacedat the same position with a semiconductor laser chip 71 b having awavelength of approximately 780 nm, the laser beam with a wavelength ofapproximately 780 nm will exit at an offset from the beam-shaping prism74, as indicated by a dotted line 76, and the beam will enter the focuslens 75 diagonally. This happens because the refraction index willdecrease for longer wavelengths in standard optical materials.

[0047] In FIG. 7(b), the collimating lens 73 and the focus lens 75 fromFIG. 7(a) are omitted for convenience. The laser beam 72 a is the beamwith the wavelength of approximately 660 nm from the semiconductor laserchip 71 a. The dotted line 76 shows the exit direction of the laser beamwhen the semiconductor chip 71 b with a wavelength of approximately 780nm is put in the place of the semiconductor laser chip 71 a. When thesemiconductor laser chip 71 b with a wavelength of approximately 780 nmis put in the place of the semiconductor laser chip 71 a with awavelength of approximately 660 nm, the dispersion characteristics ofthe beam-shaping prism 74 cause the exit angle of the laser beam withthe wavelength of approximately 780 nm to be offset as indicated by thedotted line 76. As shown in the figure, the semiconductor laser chip 71b is rotated to the right (clockwise) from the position of thesemiconductor laser chip 71 a or is shifted to a position close to aline extending from the exit beam 72 a of the beam-shaping prism 74.With this arrangement, the laser beam 72 b with the wavelength ofapproximately 780 nm is shifted so that the offset caused by thedispersion characteristics of the beam-shaping prism 74 is canceled out,and the offset in the entry angle to the focus lens is reduced.Conversely, if the semiconductor laser chip with the wavelength ofapproximately 780 nm is placed at the position indicated by the dottedline 71 c, the laser beam will be offset as shown in the dotted line 72c in a direction where the exit angle offset caused by the dispersioncharacteristics of the beam-shaping prism 74 is increased, and theoffset in the entry angle to the focus lens is increased.

[0048] Based on the above, it is possible to make both laser beams haveroughly the same exit angles from the beam-shaping prism 74 by shiftingthe semiconductor laser chip 71 b having the wavelength of approximately780 nm appropriately from the semiconductor laser chip 71 a having thewavelength of approximately 660 nm.

[0049]FIG. 7(c) shows an optical system of an optical head equipped withthe beam-shaping prism 74. As in FIG. 7(b), the collimating lens 73 andthe focus lens 75 are omitted to simplify the discussion. The laser beam72 a is the beam with the wavelength of approximately 660 nm from thesemiconductor laser chip 71 a. The dotted line 76 shows the exitdirection of the laser beam when the semiconductor chip 71 b with awavelength of approximately 780 nm is put in the place of thesemiconductor laser chip 71 a. When the semiconductor laser chip 71 b isshifted to a position closer to a line extending from the exit beam 72 afrom the beam-shaping prism 74 than the position of the semiconductorlaser chip 71 a, i.e., its position is rotated counterclockwise to theposition 71 b in FIG. 7(c), the laser beam 72 b with the wavelength ofapproximately 780 nm is shifted in a direction that cancels out the exitangle offset generated by the dispersion characteristics of thebeam-shaping prism 74. As a result, the shift in the entry angle to thefocus lens can be reduced. Conversely, placing the semiconductor laserchip with the wavelength of approximately 780 nm at the positionindicated by the dotted line 71 c causes the shift in exit angle of thelaser beam to increase due to the dispersion characteristics of thebeam-shaping prism 74, and the shift in the entry angle to the focuslens increases.

[0050]FIG. 7(d) shows an optical system of an optical system equippedwith the same beam-shaping upward prism 10 as in the embodiment fromFIG. 1. The collimating lens 9 and the focus lens 15 from FIG. 1 are notshown in this figure. As with FIG. 7(c), the laser beam 72 a is the beamwith the wavelength of approximately 660 nm from the semiconductor laserchip 71 a. The dotted line 76 shows the exit direction of the laser beamwhen the semiconductor chip 71 b with a wavelength of approximately 780nm is put in the place of the semiconductor laser chip 71 a. When thesemiconductor laser chip 13 b is disposed at a position closer to theextension line of the refracted beam refracted inside the beam-shapingupward prism 10 compared to the semiconductor laser chip 13 a, i.e., atthe position 13 b in the figure, the laser beam 12 b with the wavelengthof approximately 780 nm is shifted in a direction that cancels the exitangle offset generated by the dispersion characteristics of thebeam-shaping upward prism 10, and the offset in the entry angle to thefocus lens can be reduced. Conversely, if the semiconductor laser chipwith the wavelength of approximately 780 nm is positioned at dotted line13 c, the laser beam will travel as shown in dotted line 12 c. Theoffset in the exit angle generated by the dispersion characteristics ofthe beam-shaping upward prism 10 will be increased and the offset of theentry angle to the focus lens will be increased.

[0051] As described above, the position of semiconductor laser chipshaving different wavelengths can be set up as appropriate so that theentry angles to the focus lens 75 are roughly identical.

[0052] Also, the above points show that the semiconductor laser chipswith different wavelengths should be placed at positions near the exitbeam positions when the beams from the semiconductor laser chips travelfrom the exit side of the beam-shaping prism 74 or the beam-shapingupward prism 10 to the collimating lens 73, i.e., when the laser beamsare projected in reverse.

[0053] By arranging the semiconductor laser chips with differentwavelengths in this manner, the laser beams from the semiconductor laserchips can be beamed to the focus lens within an entry angle tolerancerange. The entry angle tolerance range of the focus lens will varyaccording to focus lens, so the semiconductor laser chips will have tobe positioned so that they fall within the tolerance range of the focuslens used.

[0054] The following is a description of a specific shape of thebeam-shaping upward prism 10 used in this embodiment, with references toFIG. 8.

[0055]FIG. 8 is a plan drawing showing the structure of a beam-shapingupward prism of an optical head according to the present invention. Thematerial of the beam-shaping upward prism 10 shown in the figure is astandard vitreous material referred to as “BK7”. The vertex angle θformed between an entry/exit surface 10 a and a reflection surface 10 bis 13.410 degrees. The angle φ formed between the reflection surface 10b and a surface 36 of the optical head case is 31.768 degrees. The laserbeam 12 a with 660 nm wavelength from the semiconductor laser chip 13 aenters the entry surface 10 a of the beam-shaping upward prism 10 from ahorizontal direction parallel to the case surface 36. Then, the laserbeam 12 a enters the beam-shaping upward prism 10 at an angle of 71.641degrees relative to the normal to the entry/exit surface 10 a, and isrefracted and travels downward. It is then reflected by the reflectionsurface 10 b and travels upward and exits the beam-shaping upward prism10 at an angle of 18.359 degrees relative to the normal of the entrysurface 10 a. Thus, the direction of the laser beam 12 a isperpendicular to the optical head case surface 36. At the same time, thewidth of the laser beam, which is 1.5 mm when it enters the entrysurface 10 a, is increased by a factor of approximately 2.4, to 3.6 mm,after it exits. If a laser beam with a wavelength of 780 nm is projectedhorizontally and parallel to the case surface 36, the path of theimaginary laser beam corresponding to the dotted line 76 from FIG. 7would be titled 0.106 degrees to the left of the page from the exitangle of the laser beam 12 a due to dispersion.

[0056] If the semiconductor laser chip 13 a and the semiconductor laserchip 13 b disposed on the semiconductor substrate 24 in the two-lasermodule 8 shown in FIG. 5 are disposed so that the light-emitting pointsare separated by 350 microns, the focal distance of the collimating lens9 shown in FIG. 1(c) is 7 mm, and the semiconductor laser chip 13 b ispositioned above the semiconductor laser chip 13 b as shown in FIG.7(d), then the laser beam 12 b from the semiconductor laser chip 13 b,having a 780 nm wavelength, enters the beam-shaping upward prism 10 fromapproximately 2.86 degrees above the horizontal direction parallel tothe surface 36, as shown by the dotted line in FIG. 8. The laser beam 12b will be offset by 1.17 degrees to the right in the figure from theexit angle of the laser beam 12 a. Conversely, if the semiconductorlaser chip 13 b is positioned downward from the semiconductor laser chip13 a, the laser beam with wavelength 780 nm will enter the beam-shapingupward prism 10 at an angle of 2.86 degrees below the horizontaldirection parallel to the surface 36, and the exit angle will be offsetby 1.22 degrees to the left from the exit angle of the laser beam 12 a(not shown in the figure).

[0057] Thus, as shown in FIG. 7(d), positioning the semiconductor laserchip 13 b above the semiconductor laser chip 13 a will reduce the offsetin the exit angles between the laser beam 12 a and the laser beam 12 b.

[0058] In the embodiment described above, the laser light source isformed by arranging multiple semiconductor laser chips in a row orpackaging semiconductor laser chips in the same manner. However, itwould also be possible to have multiple laser oscillator regions withdifferent wavelengths disposed on a single semiconductor laser chip asshown in FIG. 9.

[0059]FIG. 9 is a perspective drawing showing another embodiment of asemiconductor laser chip. In the figure, a laser chip 91 is formed witha semiconductor process to have two laser oscillator regions. The laseroscillator regions project a laser beam 92 a with a short wavelength anda laser beam 92 b with a long wavelength. The two-laser chip 91 can beused in place of the two semiconductor laser chips 13 a, 13 b shown inFIG. 5. For example, the laser beam 92 a can have a wavelength of 660nm, the laser beam 92 b can have a wavelength of 780 nm, the intervalbetween the light-emitting points 93 a, 93 b can be 100 microns, and thelight-emitting point 93 b can be positioned above the light-emittingpoint 93 a, i.e., the laser beam 93 a with the 660 nm wavelength isprojected parallel to the case surface 36. In this case, the laser beam92 b with the 780 nm wavelength projected from the light-emitting point93 b enters the beam-shaping upward prism 10 from an angle ofapproximately 0.818 degrees above the horizontal direction parallel tothe surface 36, as shown in the dotted line 12 b in FIG. 8. The laserbeam 92 b exiting from the beam-shaping upward prism 10 will be tiltedat an angle of 0.242 degrees to the right from the directionperpendicular to the surface 36.

[0060] Conversely, if the two-laser chip 91 is formed so that thelight-emitting point 93 b is positioned below the light-emitting point93 a, the laser beam 92 b projected from the beam-shaping upward prism10 will be offset by 0.440 degrees to the left from the directionperpendicular to the surface 36.

[0061] Thus, even with the two-laser chip 91, the exit angle offsetbetween the laser beam 92 a and the laser beam 92 b will be smaller ifthe light-emitting point 93 b is positioned above the light-emittingpoint 93 a.

[0062]FIG. 10 shows a side-view of an architecture of another embodimentof an optical disk device according to the present invention. FIG. 10differs from FIG. 1 in the two-laser module 102 and the beam-shapingupward prism 101.

[0063] Unlike the semiconductor laser chip 8 from FIG. 1, the two-lasermodule 102 is arranged so that the semiconductor laser chip 13 a ispositioned above and the semiconductor laser chip 13 a is positionedbelow in the figure.

[0064]FIG. 11 shows a plan drawing of the structure of the beam-shapingupward prism 101. The material used in the beam-shaping upward prism 101is a standard vitreous material referred to as “BK7”. The angle formedbetween a surface 101 a and a surface 101 b is 29.526 degrees and theangle formed between a surface 101 b and a surface 101 c is 20.962degrees. A reflective film is deposited on the surface 101 c. The laserbeam 12 a emitted from the semiconductor laser chip 13 a has awavelength of 660 nm and enters the surface 101 a of the beam-shapingupward prism 101 from a horizontal angle and is refracted. The refractedlaser beam 12 a hits the surface 101 b at an entry angle of 59.052degrees relative to the normal of the surface 101 b. The refractionindex of the BK7 material at a wavelength of 660 nm is 1.51374 and itscritical angle is 41.347 degrees. Since the entry angle of the laserbeam 12 a is greater than the critical angle, it is reflected by thesurface 101 b. The laser beam 12 a is then reflected by the surface 101c and hits the surface 101 b again. However, this time it enters at aperpendicularly so that it passes through the surface 101 b and exitsthe beam-shaping upward prism 101. The beam-shaping upward prism 101allows the path of the laser beam 12 a to be bent at a right angle whilealso increasing the width of the laser beam by a factor of approximately2.2. If, with the beam-shaping upward prism 101 shown in FIG. 11, alaser beam with a wavelength of 780 nm is projected at the samehorizontal angle as the laser beam 12 a, the dispersion of thebeam-shaping upward prism 101 will cause the laser beam to be shifted tothe left on the figure by 0.14 degrees compared to the exit angle of thelaser beam 12 a. If a laser beam 12 b with a wavelength of 780 nm isprojected at an angle shifted upward in the figure by 0.306 degrees fromthe horizontal angle of the laser beam 12 b, as shown in FIG. 11, it canexit the surface 101 b perpendicularly as in the laser beam 12 a. Thus,as in FIG. 10, the semiconductor laser chips 13 a, 13 b in the two-lasermodule 102 should be arranged so that the semiconductor laser chip 13 ais positioned upward in the figure and the semiconductor laser chip 13 bis positioned downward in the figure.

[0065] As described above, the present invention uses the differentdispersion characteristics of a beam-shaping prism for different laserbeam wavelengths to prevent coma aberrations in the laser spotlights forthe laser beam for at least one of the wavelengths.

[0066] Also, according to the present invention, in optical heads thatrecord or playback information from or to an optical information mediumusing multiple laser light sources, an optical head and an optical diskdevice using the same can be provided that does not require new,expensive optical parts, that tends not to generate aberration in laserbeams from semiconductor lasers disposed away from the optical axis,that allows information to be recorded, and that can be formed with athin design.

[0067] According to the present invention, aberration generated by laserbeams can be reduced in cases where laser light sources with multiplewavelengths are used.

What is claimed is:
 1. An optical head comprising: laser light sourcesemitting a plurality of laser beams with different wavelengths; meansfor converting beam width having dispersion characteristics so that saidplurality of laser beams emitted from said laser light sources exits atdifferent angles when said plurality of laser beams enters at identicalangles, and converting beam widths of said plurality of laser beams; andmeans for optically focusing focusing said plurality of laser beamsexiting from said beam width converting means to an optical spot on anoptical information medium; wherein said laser light sourcescorresponding to said laser beams are positioned in the vicinity of apath of a laser beam projected from an entrance side of the beam widthconverting means when the plurality of laser beams is entered into anexit side of the beam width converting means.
 2. An optical head asdescribed in claim 1 wherein said laser light source is positioned sothat said plurality of laser beams emitted from said plurality of laserlight sources enters said optical focusing means within an entry angletolerance range.
 3. An optical head as described in claim 1 wherein saidbeam width converting means is a refracting-type means for convertingbeam widths converting beam widths through refraction.
 4. An opticalhead comprising: laser light sources emitting a plurality of laser beamswith different wavelengths; means for converting beam width convertingbeam widths of said plurality of laser beams; and means for opticallyfocusing focusing said plurality of laser beams exiting from said beamwidth converting means to an optical spot on an optical informationmedium; wherein: said beam width converting means has dispersioncharacteristics so that said plurality of laser beams emitted from saidlaser light sources exits at different angles when said plurality oflaser beams enters at identical angles; and said laser light sources arearranged in a sequence determined by wavelength in order to reduceshifting in exit angles caused by said dispersion characteristics whensaid laser beams emitted from said plurality of laser light sources exitfrom said beam width converting means.
 5. An optical head as describedin claim 4 wherein: said beam width converting means is arefracting-type means for converting beam widths converting beam widthsthrough refraction; and said plurality of laser light sources isarranged so that said laser light sources with longer wavelengths arepositioned closer to an extension line of a refracted beam created bysaid refraction of said beam width converting means.
 6. An optical headas described in claim 5 wherein said refracting-type beam widthconverting means is a prism.
 7. An optical head comprising: a pluralityof semiconductor laser chips having different wavelengths; a collimatinglens forming parallel beams from a plurality of laser beams emitted fromsaid semiconductor laser chips; means for optically focusing focusingsaid plurality of laser beams on said optical information medium as anoptical spot; and a beam-shaping prism expanding a width of said laserbeams in a direction in which said semiconductor laser chips arearranged; wherein said semiconductor laser chips with longer wavelengthsare positioned closer to an extension line of a beam exiting from saidbeam-shaping prism.
 8. An optical head as described in claim 7 wherein:said beam-shaping prism includes a reflective surface; and semiconductorlaser chips with longer wavelengths are positioned toward a reflectiveside of said beam-shaping prism.
 9. An optical head as described inclaim 8 wherein said beam-shaping prism is positioned below said opticalfocusing means.
 10. An optical disk device comprising: an optical headincluding: laser light sources emitting a plurality of laser beams withdifferent wavelengths; means for converting beam width having dispersioncharacteristics so that said plurality of laser beams emitted from saidlaser light sources exits at different angles when said plurality oflaser beams enters at identical angles and converting beam widths ofsaid plurality of laser beams; and means for optically focusing focusingsaid plurality of laser beams exiting from said beam width convertingmeans to an optical spot on an optical information medium; wherein saidlaser light sources corresponding to said laser beams are positioned inthe vicinity of a path of a laser beam projected from an entrance sideof the beam width converting means when the plurality of laser beams isentered into an exit side of the beam width converting means; wherein: alaser beam from said optical head is projected on said opticalinformation medium; a laser beam reflected from said optical informationmedium is projected onto a plurality of optical detector elements; and asignal electronically converted by said plurality of optical detectorelements is used to provide a control signal and an information playbacksignal.
 11. An optical disk device comprising: an optical headincluding: laser light sources emitting a plurality of laser beams withdifferent wavelengths; means for converting beam width converting beamwidths of said plurality of laser beams; and means for opticallyfocusing focusing said plurality of laser beams exiting from said beamwidth converting means to an optical spot on an optical informationmedium; wherein: said beam width converting means has dispersioncharacteristics so that said plurality of laser beams emitted from saidlaser light sources exits at different angles when said plurality oflaser beams enters at identical angles; and said laser light sources arearranged in a sequence determined by wavelength in order to reduceshifting in exit angles caused by said dispersion characteristics whensaid laser beams emitted from said plurality of laser light sources exitfrom said beam width converting means wherein: a laser beam from saidoptical head is projected on said optical information medium; a laserbeam reflected from said optical information medium is projected onto aplurality of optical detector elements; and a signal electronicallyconverted by said plurality of optical detector elements is used toprovide a control signal and an information playback signal.
 12. Anoptical disk device comprising: an optical head including: a pluralityof semiconductor laser chips having different wavelengths; a collimatinglens forming parallel beams from a plurality of laser beams emitted fromsaid semiconductor laser chips; means for optically focusing focusingsaid plurality of laser beams on said optical information medium as anoptical spot; and a beam-shaping prism expanding a width of said laserbeams in a direction in which said semiconductor laser chips arearranged; wherein said semiconductor laser chips with longer wavelengthsare positioned closer to an extension line of a beam exiting from saidbeam-shaping prism; wherein: a laser beam from said optical head isprojected on said optical information medium; a laser beam reflectedfrom said optical information medium is projected onto a plurality ofoptical detector elements; and a signal electronically converted by saidplurality of optical detector elements is used to provide a controlsignal and an information playback signal.